A photon scanning microscope, detecting evanescent light localized in an area smaller than the wavelength of light on a surface of a material for measuring the shape of the material, is known as an ultra-high resolution optical microscope having the resolution exceeding the diffraction limit of a conventional optical microscope.
If the sample surface is irradiated from the back surface of a sample 1 under a condition of total reflection, the field of evanescent light is generated depending upon the shape of the sample surface.
With the photon scanning microscope, the strength of the evanescent light is detected by a light probe 3 having a detection end 2 having an aperture of a size on the order of the wavelength of the evanescent light, as shown for example in FIG. 2, for producing the resolution exceeding the diffraction limit of the conventional optical microscope.
The resolution of the photon scanning type microscope is determined by the effective aperture diameter of the light probe, on the other hand, since the strength of the evanescent light is decreased exponentially with the distance from the sample surface, the aperture diameter of the light probe can be equivalently reduced simply by sharpening the end of the light probe.
The light probe shaped as shown in FIG. 1 has a cladding diameter D (approximately 60 .mu.m) much larger than the length L of the detecting end 2 (approximately 2 to 6 .mu.m). Thus there was a risk that an edge portion 4 of the cladding be struck against the surface of a sample 1 thus destructing the sample or the optical probe 5 itself.
The inventors hereof have proposed an optical fiber for overcoming these problems and filed JP Patent Application Nos. 5-291829, 6-53626 and 6-55697 and PCT/JP 94/00906. In these applications, a core protruded from a cladding on one end of an optical fiber is sharpened to form a tapered detection end, on the proximal end of which is formed a small-diameter portion formed by reducing the diameter of the cladding, or both the detection end and the cladding are tapered at the distal ends, for preventing the edge portions of the cladding from impinging against the sample surface.
Moreover, with the above-described photon scanning tunneling microscope, since the evanescent light is extremely small in strength, it is necessary to avoid the effect of the scattered light to raise the detection efficiency of the evanescent light. For example, it may be envisaged to form a light-shielding coating layer on the surface of the detection end of the light probe and to form an aperture on the order of the wavelength of the detection light on the distal end of the evanescent light to be detected at the distal end of the light probe. The evanescent light to be detected is caused to be incident only at the aperture for shielding the light irradiated on an area other than the aperture for diminishing the effect of the scattered light.
To this end, with the optical fiber according to the above-mentioned JP patent applications, the optical fiber is rotated in a vacuum about its own axis and the vapor of a light-shielding material is supplied from the lateral or rear side of the detection end for forming a coating layer of the light-shielding material on the surface of the detection end and for exposing the distal end of the detection end via the coating layer of the light-shielding material for forming an aperture.
However, there are involved difficulties in uniformly forming the light-shielding coating layer and informing a small-sized aperture on the order of the light wavelength or with good reproducibility.
Meanwhile, there is known an optical fiber sensor on the distal end of which is deposited a functional material which is changed in optical characteristics depending on the surrounding environment, such as fluorescent substance or reagent. Such an optical fiber sensor is shown for example in FIG. 3 in which a light-shielding coating layer 7 is formed on the surface of a tip formed by sharpening a core, the light-shielding coating layer 7 is formed on the surface of the tip 6 and a functional substance 8, such as the fluorescent substance or the reagent, is affixed on an aperture formed by exposing the distal end of the tip 6 from the light-shielding coating layer 7 (eds. W. Pohl and D. Courjon, Near Field Optics (Book) 1993, pages 17 to 24).
In such optical fiber sensor 5, the functional substance 8 at the distal end of the tip 6 is changed in optical characteristics, such as light emission spectrum or in light absorption spectrum, depending on environmental conditions, such as intensity of the ambient light or pH values. These changes in the optical characteristics are detected at the opposite end of the tip 6 for detecting the surrounding environment of the tip 6. It is possible with the optical fiber sensor 5 to reduce the size of the functional substance 8 deposited on the distal end of the tip 6 for improving the spatial resolution for detection and expediting the response as compared to the conventional electrical sensor.
With such optical fiber sensor, there are involved difficulties in depositing the functional substance on the distal end of the optical fiber with a high exfoliation strength for improving durability.